As a technique of enhancing the hardness and rigidity of rubber compositions to improve the handling stability, Patent Literature 1, for example, discloses the addition of a phenol resin and a curing agent for curing the resin into a rubber composition, followed by performing a crosslinking reaction of the phenol resin while vulcanizing the rubber composition in the vulcanization stage. Alternatively, it has been known that microfibrillated plant fibers such as cellulose fibers are compounded into a rubber composition.
However, if a phenol resin or microfibrillated plant fibers are compounded as a reinforcing material into a rubber composition, the energy loss (heat build-up) caused when the rubber composition is deformed tends to increase due to friction between the rubber component and the reinforcing material as well as friction between the reinforcing materials. Therefore, when the rubber composition containing the reinforcing material is used in a tire, which is a typical application of rubber compositions, the rolling resistance tends to increase to cause a reduction in fuel economy.
Microfibrillated plant fibers are hydrophilic materials and thus are poorly compatible with the rubber component. Accordingly, if microfibrillated plant fibers are compounded into a rubber composition, the microfibrillated plant fibers are insufficiently adhered to the rubber component, and the rubber composition is likely to be broken on its deformation. Further, energy loss in the interface between the rubber component and the microfibrillated plant fibers tends to be caused.
As described above, if a phenol resin or microfibrillated plant fibers are used as a reinforcing material, the energy loss will be increased, and hence further improvement is needed on this point. Microfibrillated plant fibers are also poorly adhered to the rubber component and hence further improvement is needed on this point. Therefore, unless these properties are improved, they are difficult to apply to tires for various uses and in particular those used for a long period of time under harsh conditions. In recent years, there has been proposed a technique of enhancing the compatibility of pulp with the rubber component by chemically treating pulp with a silane coupling agent containing an amino group. However, this technique requires a chemical reaction process and therefore a simpler technique is desired.